Dennis Root said he spoke with Zimmerman, standing trial in the shooting death of Florida teen Trayvon Martin, and reviewed evidence in the case before testifying. Zimmerman, 29, claims he shot Martin in self-defense after the teen attacked him and slammed his head into a sidewalk.

During Root's testimony, prosecutors and defense attorneys used a mannequin to re-create portions of the fatal altercation between Martin and Zimmerman.

Root said he believed the screams heard in the background of a 911 call placed by a neighbor indicated a "high level of stress, high level of fear." He testified that the length of time the altercation lasted - the screams span about 40 seconds on the call before the gunshot- could seem like an "eternity" to someone involved in the fight.

"40 seconds of time, that's a very long time to be involved in any kind of physical altercation," Root said. "That most certainly plays into the decision making of any individual involved."

Who is screaming on the call has been a key point of controversy in the case.

On cross-examination, prosecutor John Guy asked Root whether he would scream if he was in a fight with someone and he realized the person had a gun.

"My first instinct as the aggressor is not going to be to scream to help," Root said. "It's to go for the weapon that's going to help me continue my aggressions."

Zimmerman has said that he believed Martin was reaching for his gun before the fatal shot.

Root also testified that he believed Martin was in better physical shape than Zimmerman and that Zimmerman wasn't any athlete. "He would find himself lacking when compared to Mr. Martin," Root said.

He said that carrying a gun with a round in the chamber - as Zimmerman had been the night of the fatal struggle - was the "best way to carry a firearm."

"Under stress or under any type of threat, you may not have the time to work it, but more importantly, you may not have the memory to say, 'Oh, I've got to load a round into the chamber,'" Root said.

Responding to questions from prosecutors, Root said that he had never before testified before a jury as an expert in a criminal case. He said he knew there would be publicity in the case, but he got involved because "I'm dedicated to finding the truth."